The present invention is related to the field of printed circuit board manufacturing.
In the manufacture of printed circuit boards, especially relatively thick circuit boards such as used for backplanes and mid-planes in electronic equipment, it is known to perform counter-boring or back-drilling of selected plated through holes (vias) that are used to make signal connections between different layers. If the connection formed by a via is near one surface of the circuit board, then the part of the via extending away from the connection to the other surface of the circuit board forms a “stub” that can cause undesirable effects on high-frequency signals. By back-drilling such vias to remove the stub, the quality of the signals carried by these via connections can be improved.
There are known methods of verifying that back-drilling is being performed to the correct depth. These methods include the use GO/NO-GO depth gauges to verify drill depth, either on a representative sample of back-drilled vias or on all back-drilled vias. This method only determines the depth of the drilled hole without any indication of the hole's relationship to the target signal layer, whose depth may vary considerably due to normal manufacturing tolerances. As an alternative, time-domain reflectometry (TDR) testing can be used to identify gross changes in the via stub length, but with only limited resolution.
In theory, back-drilling is a simple manufacturing process, but it has not been readily verifiable. The target layer's actual depth in the PCB stackup can vary substantially due to variations in raw material thickness and processing. Drilling too deep can ruin the board. Drilling too shallow may leave too much via stub, increasing the risk of latent signal-integrity issues which could cause the system to exhibit transient errors or to fail when upgraded to higher signaling rates, possibly after being in service for many years. Standard continuity checking only confirms that the counter-bore operation did not create any open circuits, and provides no information regarding the depth of the back-drilling with respect to the target signal layer.